Python data and variable types
I am a great believer in “blog your homework”. By homework, I mean (in this case) what I am learning about Python for my visual novel. In addition to my learning, I have two friends who are or are about to start using Python. This is a primer for all (3) of us.
Those who know their Python are invited to chip in if I missed anything or got something wrong. I’m here to learn too. There may be parts of this with updates from future me who just learned a thing.
What are variables
A quick refresher: Python, like most programming languages, has two things that are data-related:
VariablesFunctions aka Mthods in some contextsVariables hold data and functions do something with data. 2+3=5 is a function where 2 and 3 are variables and 5 is the new variable that + made. This is what it looks like in code.
def adding(var,othervar): return var+othervarmyvar = 2myothervar = 3myanswer = adding(myvar,myothervar)print(myanswer) # 5
Types of variable – in general
Before we get to the depths of Python-specific stuff I want to revise variables in general.
Most of this will also apply to things called constants which are values that cannot (or should not) change. Constants are a special case that I shall only tough on a bit.
Boolean (bool)
Booleans are the most primitive variable type. They can be True or False and that is it. This should relate to binary 1 or 0 but different languages handle them differently. On the whole, booleans are [1] or [NOT 1] but can also be [0] and [NOT 0] because screw consistency, I guess.
Most languages have True and False as core contestants and it is best to use those. Python is no exception.
Later note: False and 0 are considered the same in Python.
Integer (int)
Whole numbers (called integers by maths and computer science people) are something you will probably work with at some point. These are the variables in my silly example of variables vs functions.
The number 42 is an int but 4.2 is not. That dot is what is different.
Reals, floats, doubles and their friends
These are all variables of various and fairly arbitrary precision. For now, the difference between these is not important. It might come up later but just think of numbers with a decimal point. That 4.2 (four point two) from earlier belongs here.
If you cast (converted) 42 into one of these floating point types, you would get 42.0 or more likely 42.000000…
Casting floats into ints gives you weird behaviour and maths errors. For example, push 3.14 (three point one four) and 4.2 through an adding function that outputs only ints, and you will get 7 (instead of 7.34).
Personal peeve side story: When you see a number like 3.14 and you say “three point fourteen” please understand that I may wish to percusively educate you with a large plank of wood. This is because this number is three point one four. The only time you may call it three point fourteen if it is part of a version number like this: 3.14.0.2289. Otherwise, please slam your head in the door until you can say it properly.
Char
A single character like a letter or a number (as text). This will come up in database stuff as a common field type is varChar (variable characters).
Array
An array is basically a list of variables. The details vary between languages and Python gets fussy about them.
An array has two parts – the index and the value.
Here is a PHP example because I know PHP better and Python gets a bit complicated.
$myarray = array('bannas','apples',12);$myarray[0] = 'Chips';$myarray[] = 'boats';$myarray['foo'] = 'bar';print_r($myarray);/*array( 0 => 'chips', 1 => 'apples', 2 => 12, 3 => 'boats', 'foo' => 'bar')*/
Arrays index by number starting from 0 (most of the time). In Python, a negative index number is the point that number from the end. In PHP a negative index is just another index.
Some arrays can have text (strings) as an index. In PHP that last array element also has a secret index value of 4.
Try not to worry about this, things only get stranger from here.
Strings
We mentioned strings. Strings are characters all hanging out together in a single variable. This entire post is stored as a complicated string.
Just to confuse you, some languages like Ruby don’t really have strings, they have arrays of char. Thus, you can grab any character you want from the string. A fair amount of languages now allow you to treat strings as pseudo-arrays (treat them like arrays even though they are strings).
Strings have encodings. This is because there are a lot of languages with a lot of different characters. And now there are emojis as well. While this should not matter until you start processing external data or using databases, it will come up eventually. If in doubt, I use UTF-8. However, check with your data provider or database person because mismatched encodings give funny-looking results.
Python defaults to UTF-8.
Weak or strongly typed variables
Sooner or later, you are going to come up against typing issues. For example:
myStr = "42"myInt = 42myResult = myStr+myInt # what will you get from this?
As this answer explains, in Python you will get an error. This is because Python is strongly typed and adding a number to text makes no sense.
In PHP you will get “4242” because PHP is weakly typed and you just merged the int into a string like a complete noob.
Classes (in general)
Most programming languages have the concept of a class object. A class holds variables and has methods with are just functions that belong to the class. Python is one such language. If I mention classes (and I will because Python data types are classes) this is what I am on about.
In python, you can use (dot) methods on classes.
Variable Names
It is said there are two hard problems in computer science – naming variables, invalidating cache, and off-by-one errors.
In this example, I have named the variables so as to remind you what they are. This is terrible practice for anything that is not really basic notes. Please, name your variables something that describes what it is for. Your future self and whoever inherits your code will thank you.
Don’t do this:
n = 'bob'i=22tt = '01843 123456'
You might know now what those variables are but will you always remember?
Instead, do yourself a favour and do something like this:
BobName = "Bob"BobID = 22BobTelephoneNumber = '01843 123456'
There are better ways of handling names and numbers than this but at least you won’t come across code like this and wonder what it is about.
if(i>23): tt=''
What is that code even doing?
Advanced example (skip if confusing)
As per my snark about the “good” example not being good: Assuming you made a class called person in a library called myPersonThing, you can make Bob an instance of that.
import myPersonThingBob = myPersonThing.person('Bob',22,'01843 123456')# Bob.Name = "Bob"# Bob.ID = 22# Bob.TelephoneNumber = '01843 123456'
Note: When we come to dict(), you will have yet another way to approach this.
Variable types: Python specifics
This is where the blogging my homework comes in. While the basics are already covered, its time to get crazy. I’ll start us off slow.
For reference, there is a proper list of Python’s built-in types here. You can add more via libraries but I am not covering those.
Booleans
Booleans work exactly as you should expect them to. They are True or False. 0 counts as False. Logical.
AreYouAlive = Trueif AreYouAlive: breath()else: rot()
A better example:
UserCredits = GetUserCredits(Bob)if (UserCredits>0): print("Welcome citizen")else: print("Begone worthless fool")
The boolean here is hidden inside the logical test (UserCredits>0). But that is a topic for another long post. The outcome is unknown but depends on what the GetUserCredits() function returns. Judging from the code, we are expecting a number.
String (Str)
The str type is exactly what you would expect.
myStr = "42"print(type(myStr))
You will get an answer that this is a class of type str. You remember I mentioned classes, string is a class (technically str is a class).
This allows such behaviour as:
myStr = "foo bar baz boo"myNewStr = myStr.capitalize()print(myNewStr) # Foo bar baz boo
The capitalize() (note the spelling) method gives you a copy with the first letter capitalised.
Other methods you may need for working with other data encodings may include this one:
myStr = "foo bar baz boo"mySafeStr = myStr.encode('mac_latin2','ignore')
This will give you a copy of the string converted (as best as Python can) to mac_latin2 bytes. Here is a list of all encodings.
You can join strings together like this:
myStr = "foo"myOtherStr = "bar"myNewStr = myStr+' '+myOtherStr # we add the two with a space in the middle (that's a string too)print(myNewStr) # foo bar
Numbers
Python, it turns out, has three number types.
intfloatcomplexThat third one was a surprise to me.
Note: There is a fourth thing called a range but I covered it in the lists and arrays section like a fool.
Int
We covered integers in the general theory section. This is a whole number.
Here is how you might use them.
def adding(var,othervar): return var+othervarmyvar = 2myothervar = 3myanswer = adding(myvar,myothervar)print(myanswer) # 5
A better function would check the inputs are a number type. Do not copy this for any real-world code.
Float
When you encounter reals, floats, doubles and their friends in database stuff, they are all floats in Python. You should remember (unless you skipped it) that floats are numbers with a decimal point (also called a floating point number).
The number we all know and love – 3.1415926535… – is a float of (theoretically) infinite length. The longer your approximation of pi, the more accurate (and time-consuming) your maths will be.
42 as a float is 42.000…
If you don’t care about the precision, you can cast floats into ints but it is probably not a great idea.
myFloat = 3.14159265print(int(myFloat)) # 3
Here is a whole nerdy discussion about casting floats to ints for those who want to do that sort of nonsense.
Complex (skip if confusing)
Complex numbers are a fun part of maths that can make certain operations faster. For example, real-time radar. You might want to use them for making Mandelbrot fractal images.
In regular mathematics complex numbers have a real and a complex part like this 5+2i. The complex value i is used. the number i is what you would get if there was an answer to what is the square root of -1 (minus one).
In Python for reasons I am yet to work out, complex examples use i for the real part and j for the imaginary part. It is like someone read about the quaternions but only half remembered it.
The complex is two numbers together and can be generated by the function complex() like this:
myComplexNumber = complex(1,2) # 1+2imyComplexNumber = complex(1j) # 0+1imyComplexNumber = complex("9i+1j") # 9+1i
For most of us, complex numbers are things we will not have to deal with. Unless you are doing some serious science or intensive recreational mathematics.
Are lists arrays?
Remember when I told you about arrays? Ah, the days of easy stuff. Get ready for some Python-specific confusion. This is where I trip up coming from PHP to Python.
Allow me to introduce you to list, tuple, range, dict, set and frozen set. Yeah, I see that look. I had that same look on my face because I thought I would instantly understand this stuff.
x = ["apple", "banana", "cherry"] # a listx = ("apple", "banana", "cherry") # a tuplex = range(6) # a rangex = {"Name" : "Bob", "ID" : 22, "TelephoneNumber" : '01843 123456'} # a dictx = {"apple", "banana", "cherry"} # a setx = frozenset({"apple", "banana", "cherry"}) # a frozen set
Choose the wrong one and it will not behave how you were expecting. For me, I still think of these as types of arrays. It is probably better that you do not copy me.
Let’s look at them one by one.
List
This is what other languages would call a numerically index array. List items are ordered, changeable, and allow duplicate values. They are indexed numerically from 0. This is where you can try for a negative index and get not what you thought you would get. At least I did.
x = ["apple", "banana", "cherry"]print(x[0]) # appleprint(x[-1]) #cherryprint(x[2]) # also cherryprint(x[1]) # banana
Did you want to add a new item to your list? Sorry, you will have to make a new list that is one entry bigger.
Tuple
Tuples are a bit hard to explain. Python is the only place I have ever encountered them.
After a bit of reading, I can tell you this.
A tuple is like a list in that they are ordered and allow duplicates except the order cannot change and the values inside cannot change. You use round brackets to make them.
x = ("apple", "banana", "cherry")
Here’s a link if that was a bit unclear.
Range
The range is probably not really meant to be here. It is an ordered range of numbers.
The range function takes up to three arguments (values needed to use it). Like this: range(start, stop, step) – unless you only supply one – let me explain that.
x = range(6) # 0 1 2 3 4 5 - stop givenx = range(3, 6) # 3 4 5 - start and stop givenx = range(3, 20, 2) # 3 5 7 9 11 13 15 17 19 - start, stop, and step given
- start: Where you start from if you only give one argument, it will not be this one.
- stop: Where you stop (not included)
- step: Defaults to 1 but can be anything. To use this you need three arguments.
Another example. Say you want all the multiples of seven less than one hundred.
x = range(7,100,7)for n in x: print(n)
That prints only the numbers we want.
I guess you get a sort of ordered tuple?
Dictionary aka dict
This is what I, as a PHP programmer, would call a string-indexed array. As of newer versions of Python dictionary types are also ordered (they have a numerical index).
Dicts (try saying that out loud at work) are key-value pairs. Like field and value on a form. Each key must be unique.
Bob = { "Name" : "Bob", "ID" : 22, "TelephoneNumber" : '01843 123456'} # make the dictBob['xp']=400# you can also have more than one value by adding a tuple or a list or another dictBob['awards']=['Best Coder','Worst Farts','Employer of the year 1066'] # this list is now an entry in the dict
Note: there is no comma after the last entry in the initial dictionary definition.
In my opinion, if you need an arbitrary blob of data, dicts are usually what you want. That said, you will discover custom classes soon enough and you may want to use those.
Sets
As I write this, I haven’t a clue what sets are but I really hope they relate to set theory or are numerical random access changeable array things.
Nope, sets are more like set theory than data theory.
A set is an unordered unchangeable grouping. You cannot edit the items but you can remove items and add items. So, get item, remove it, make changes to the got item, and put it in again.
x = {"apple", "banana", "cherry", True, 99}print(len(x)) # 5x.discard("banana")print(len(x)) # 4#There is also a remove method but it throws an error if the thing is not there.x.remove("banana") # you just crashed the program noob
You can do some other stuff with sets. Popping for example.
offering = {"space", "adventure", "really wild things"}x = offering.pop()print(x) # you will get one of the things in offering.fixedIt = x.capitalize()offering.add(x) # put it back
We can also add other listy groups into a set
offering = {"space", "adventure", "really wild things"}avoiding = {"milk","cookies","an early night"}offering.update(avoiding)print(offering) # {"space", "adventure", "really wild things","milk","cookies","an early night"}
We can add those tuples and lists in the same way.
offering = {"space", "adventure", "really wild things"}avoiding = ("milk","cookies","an early night")offering.update(avoiding)print(offering) # {"space", "adventure", "really wild things","milk","cookies","an early night"}
Pay close attention – those last two may look nearly identical but they are not the same even if the output is. These things look like other things issues will trip you up if you are not careful.
Frozen sets
x = frozenset({"apple", "banana", "cherry"}) # a frozen set
A frozen set is like a set but you cannot change it. I’d have called that a constant but no one asked me.
And now we get into the weeds
Here we look at four odd fellows – bytes, bytearray, memoryview, and NoneType sounds a lot like The Null Hypthoisis if you ask me.
Each of these I learned about so I could write them down. They were all new to me.
Bytes
For some reason, you want the string as raw data (make sure you pick the encoding). You cannot change data of this type.
This is the type you get if you convert a nice well-behaved string into a specific encoding. You might want to do this to save a string in a format that is not UTF-8 because the people giving you the data are unrefined noobs or something.
Bytesarray
Yes, at last, an honest-to-goodness array. This is an array of bytes. You can change the array.
I haven’t a clue what you would do with one of these. Maybe manipulate some strangely encoded text?
Memoryview
This is a memory-safe way of looking into memory. NGL, this is all new to me. Here’s a post about it.
NoneType
It turns out that NoneType represents the distinct lack of anything. This SE answer explains it better than I could.
It is a good idea to check a return object against NoneType before using it.
obj = myUnreliableFactoryMethod(foo,bar)if obj is None: # throw an error or somethingelse: # do something clever
And that’s a wrap
We have looked at all the built-in variable types and the functions that make them. I half-arsed a few of the more esoteric ones at the end because my brain is melting.
I think I learned something. I hope you did too.
Feel free to pass judgment, offer corrections, or expand on something. I’m here to learn. Also, please ask questions if I was unclear or if you want to know more. If I can answer, I will. Otherwise, I will go away and learn the answer for you.
Welcome to your new life as a Python geek.
Syndicated to: #blogYourHomework #cherry #Python #There #variables #CodingAndDevelopment
Read the whole post
Thanet Creative
Matthew Brown
